While antibiotic use has been identified as the most likely selective pressure driving the increasing frequency of resistance genes, very few studies have experimentally measured the selective pressures they impose, the effects of varying the duration of antimicrobial exposure, and the effects of simultaneous exposure of multiple antibiotics on bacterial populations. Understanding basic evolutionary dynamics of bacteria as they respond to a variety of antibiotic resistance selection pressures will provide an experimental basis for developing antibiotic prescription guidelines that may contribute to maintaining the usefulness of currently available antibiotics. The overall goal of this project is to experimentally determine the fitness effects of expressing antibiotic resistance determinants. The specific aims of this study are: 1. Determine fitness effects associated with carriage of the more commonly encountered resistance genes in the presence of the corresponding antibiotics. 2. Experimentally determine how temporal variation in antibiotic dosing affects the frequencies of resistance genes. 3. Experimentally determine how simultaneous application of multiple antibiotics affects the frequencies of resistance genes. These studies will focus specifically on TEM, SHV, and CTX-M 2-lactamases, which hydrolyze penicillins, cephalosporins, and other 2-lactam antibiotics. I anticipate that data produced in these experimental evolution studies will help to inform future pathogen remediation efforts. PUBLIC HEALTH RELEVANCE: Antibiotic resistance imposes an enormous health burden throughout the world. The experiments described in this proposal are anticipated to contribute to the development of practices that will reduce the occurrence of resistance.